[Amps] SB200 input SWR tuning

[Tom W8JI]

> I had a FL-2100B amp and the way they said to tune the 
> input is to
> disconnect the high voltage lead from the transformer and 
> tune the input
> that way.

Wow!! Is that a silly idea if I ever heard one!!

When you remove HV the input impedance of the cathode driven 
tube goes way up. The grid current also goes way up.

Bad way to tune the input, good way to ruin the tubes.

The only good way to tune the input is to either remove 
filament and high voltage and shunt the filaments to ground 
with resistors that emulate the driving impedance at each 
tube cathode, or tune the thing while operating if that can 
be done safely.

73 Tom